Isotopic-Geochemical Systematics (Sm–Nd, Lu–Hf) of Neoarchean Subalkaline and Alkaline Rocks of the Keivy Structure (Kola Peninsula): Their Age and Genetic Relations

Abstract

The Neoarchean subalkaline magmatism of the Keivy structure is expressed in the formation of the volcanoplutonic latite–monzonite–granite association (LMGA). The formation of LMGA magmas is assumed to occur due to melting of metasomatically altered mafic rocks during intrusion into the lower crust of basaltic melts initial for rocks of the dike complex and gabbro–labradorite massifs. The alkaline granites associated with LMGA have a close U–Pb age but a later formation time based on the geological data. With respect to LMGA, alkali granites have increased concentrations of SiO₂, alkalis (K₂O/Na₂O = 1.1–1.4), iron (F# = 84–98%), a high agpaitic index (K_agp = 0.86–1.2), and lower quantities of TiO₂, MgO, Fe_tot, and Al₂O₃, which probably resulted from the higher degree of differentiation of their initial melts compared to LMGA.